The formation of the Dorsal nuclear gradient in the early Drosophila embryo represents the last step in the maternally encoded pathway that establishes dorsal-ventral polarity. The nuclear gradient is formed when, in response to a ventral signal, the inactive cytoplasmic Dorsal protein is dissociated from its inhibitor, Cactus, and targeted to the nucleus. These events are conserved in the larval fat body, where the Drosophila immune response occurs, and also in vertebrates in the activation of Re1 proteins. Vertebrate Rel proteins function in the development of the liver, thymus, and in hematopoiesis, as well as in the immune response. In addition, Rel proteins control the expression of viruses such as HIV. Our project focuses on the Dorsal pathway in early Drosophila embryogenesis. We propose to identify additional genes that function in the pathway, in particular, genes whose products directly interact with Dorsal and Cactus. We also propose to investigate the importance of phosphorylation for nuclear targeting, gradient formation, and the function of the Dorsal protein. We will also study the function of the Rel pathway, and the function of the rel homolog Dif in the humoral and cellular immune response.